1. Field of Invention
This invention relates to a chemical recovery process for recovering sulfur from spent pulping liquors and, more specifically, relates to recovery of sulfur dioxide from sodium sulfite, calcium sulfite, magnesium sulfite, and ammonium sulfite semichemical and chemical spent pulping liquors.
2. Description of the Related Art
For most sulfite pulping processes, a simple and economical chemical recovery system, to recycle the sulfur based chemicals, has not been developed. Spent sulfite pulping liquors are often sewered to secondary waste treatment or are burned without chemical recycling. The use of a recycling process reduces the amount of fresh sulfur dioxide raw material which must be used for the pulping, and recycling minimizes the amount of waste which must be further processed.
This invention is a recovery process for recycling sulfur dioxide from the spent liquor from sulfite chemical or sulfite semichemical processes.
Papermaking pulp is manufactured by (1) mechanical, (2) chemical, and (3) semichemical processes.
Mechanical pulping processes, (1), produce pulp by using mechanical energy to shred and separate raw material fibers.
In chemical pulping processes, (2), wood chips are cooked with chemicals in an aqueous solution at elevated temperature and pressure. Wood is comprised primarily of cellulose, lignin, and hemicelluloses. The pulping chemicals solubilize the lignin, which holds together the fibers in the wood.
The two major chemical pulping processes are the kraft process and the sulfite process. The invention is an improvement to the sulfite process.
The sulfite process uses a combination of sulfurous acid, sulfite ion, and bisulfite ion to solubilize the lignin. The chemicals used for the sulfite process are sulfites and bisulfites of calcium, magnesium, sodium, or ammonium; sulfite pulping can be carried out over a wide range of pH.
Semichemical pulping processes, (3), combine chemical and mechanical methods. Wood chips are partially softened with chemicals; the balance of the pulping is by mechanical force.
Mechanical pulping processes convert 90% to 95% of the wood into pulp.
In chemical processes, much of the lignin, the hemicelluloses, and some of the cellulose is solubilized by the chemicals. The pulp yield, based on original dry wood weight, is 40% to 55%.
The pulp yields from sulfite semichemical methods range between mechanical pulping yields and chemical pulping yields. That is, the yields range from 55% to 90%.
In each process, a portion of the original dry wood is solubilized and forms part of the spent liquor. Lignosulfonates, the reaction products of the lignin and the sulfite pulping chemicals, are produced.
Sulfite recovery systems have been developed for both semichemical and chemical pulping processes. Examples are U.S. Pat. Nos. 1,892,100, 1,973,557, 1,659,193, 2,022,872, 2,496,550, 2,642,336, 2,730,445, 2,701,763, 4,148,684, 4,241,041, 4,212,702, and 4,336,189.
None of the existing processes is similar to that of the petitioners. The petitioners' process is simpler and more economical. As can be seen from the following description of the related art, all of the competing recovery systems require a recovery furnace. Petitioner's does not.